The present invention relates to a digital signal detecting method which permits the reception of signals modulated by different modulation schemes and send signals of various symbol transmission rates and a detector therefor.
To realize future multimedia communications, there is a demand for techniques of transmitting data, speech and images over the same digital radio channel. One possible means for effectively transmitting data, speech and images by digital radio communication is to use symbol transmission rates and modulation/demodulation schemes optimum for the objects to be transmitted. In the field of mobile communication, for instance, it is to be wished that the base station offer a service of providing still pictures of television, data bank or the like, whereas the mobile station be capable of receiving such still pictures from the base station by simple operation with simple equipment as well as conducting usual voice communications. In this instance, a QPSK modulation scheme is usually employed for the voice communication but a QAM or similar multilevel modulation scheme is needed for the transmission of still pictures because of the necessity for transmitting a larger amount of information than that required for the voice communication. This requirement could be met by providing independent transmitters and receivers each corresponding to a particular modulation/demodulation scheme as shown in FIG. 1A, in which the transmitting station is provided with a transmitter group 10 consisting of, for example, QAM, PSK and FSK modulating transmitters 11, 12 and 13 and the receiving station is provided with a receiver group 20 consisting of QAM, PSK and FSK receivers. Another method is common to the above in the provision of the independent transmitters 11, 12 and 13 at the transmitting side but differs therefrom in that the receiving station is equipped with a Single receiver 21 with QAM, PSK and FSK detectors 22, 23 and 24 built therein as shown in FIG. 1B. One possible method for providing a plurality of detectors in the same radio as shown in FIG. 1B is to build therein independent detectors each designed specifically for one modulation/demodulation scheme.
At present, mobile communication services are allocated 800 and 1500 MHz bands but cannot be switched back and forth between them. If the bands can be switched by a simple operation with a simple structure, however, cochannel interference can be reduced by using the 800 MHz band outdoors and the 1500 MHz band indoors and in closed spaces through utilization of a property that the linearity of electric waves in the 1500 MHz band is higher than in the 800 MHz band.
The device configuration depicted in FIG. 1B has a plurality of independent detectors built-in, and hence it is inevitably bulky and complex. Furthermore, in the digital radio communication for transmitting data, speech and images, it is hard to instantaneously switch the independent detectors by dynamically changing the demodulating scheme and the carrier frequency. The receiver 21 quadrature-demodulates the received signal, for which it is necessary to generate a local oscillation signal synchronized with the carrier of the input received signal. In this instance, if the carrier frequency of the received signal varies from f.sub.1 to f.sub.2, f.sub.3, and f.sub.4 with the lapse of time as shown in FIG. 2A, the frequency of the local oscillation signal also needs to vary correspondingly. To meet this requirement, it is general practice in the prior art to employ such a method as shown in FIG. 2B, in which the oscillation frequency of a PLL local oscillator 25 is switched by switching means 17 to f.sub.1, f.sub.2, f.sub.3 and f.sub.4 one after another as indicated by local oscillators 25.sub.1, 25.sub.2, 25.sub.3 and 25.sub.4, then the output from the switched local oscillator and the input modulated signal are multiplied by a multiplier 18 and the multiplied output is applied to a filter 19 to obtain a base band signal. The frequency switching speed in the PLL local oscillator 25 is several milliseconds at the highest even by the use of a digital loop preset type frequency synthesizer. With such a low response speed, it is impossible to fully respond to the frequency switching during communication.
For example, when the symbol transmission rate of the received signal varies from B.sub.1 to B.sub.2, B.sub.3, and B.sub.4 with the lapse of time as shown in FIG. 2C, it is conventional that filters 26.sub.1, 26.sub.2, 26.sub.3 and 26.sub.4 for filtering the output from a quadrature demodulator are switched one after another by switching means 27 and 28 in response to the variation in the transmission rate of the received signal as depicted in FIG. 2D. Since the filters are formed by hardware, the filter switching speed cannot be increased because of transient characteristics of the filters.
It is therefore an object of the present invention to provide a digital signal detecting method and a detector therefor which enable digital communication equipment having a plurality of detecting means built-in to be used in common to pluralities of modulation/demodulation schemes, local oscillation frequencies and symbol transmission rates.
Another object of the present invention is to provide a digital signal detecting method and a detector therefor which are capable of responding fast to the switching of the modulation schemes and a change in the symbol transmission rate.